Paper coating formulations for rotogravure applications

ABSTRACT

Disclosed are paper coating formulations for rotogravure printing processes containing: a. 100 parts by weight or finely divided pigments; b. from 0.001 to 5 parts by weight of one or more substances selected from the group consisting of: mono-alkylsulfosuccinate; dialkylsulfosuccinates; sulfosuccinic acid mono-esters of ethoxylated and/or propoxylated fatty alcohols; sulfosuccinic acid di-esters of ethoxylated and/or propoxylated fatty alcohols; c. from 3 to 15 parts by weight of a polymeric acrylic binder, d. from 0.005 to 0.4 parts by weight of a dispersant.

The present invention relates to paper coating formulations forrotogravure applications, and to their aqueous dispersions.

It is well known that the surface of printing paper sheets is commonlycoated to improve the printability.

In the present text, with the expression “paper coating formulation” wemean the stratum of mixed pigments that is applied on the paper sheet tomake them smooth and glossy and with the term “coating” the procedureused to apply it on the paper sheet.

The paper which is normally used for the manufacture of magazines orother objects (paper bags, wrappers and the like) and undergoes arotogravure process (hereafter rotogravure paper) must possess, togetherwith a good printability, proper characteristics in terms of:

-   -   mechanical resistance, to resist to the high speeds of the        printing machines;    -   tint, that normally, for aesthetic reasons, it is requested to        be as white as possible;    -   ink permeability, to avoid blurs;    -   smoothness and glossiness.

The obtainment of a rotogravure paper possessing all thesecharacteristics at an optimum level is still a not completely resolvedproblem, and therefore, the whole of the properties of the paper usedfor rotogravure processes is a compromise solution.

To cite an example, the use of talc in paper coating formulationsimproves the printabilty of rotogravure paper and gives to the surfaceof the paper itself a velvet-like touch, but markedly worsens therheological characteristics of the aqueous dispersions of the papercoating formulation, not allowing to work with a high content of solids;moreover, talc, because of its greyish tint, diminishes the brillianceand the whiteness of the paper and because of its intrinsic hydrophobiccharacter requires great care in dispersion.

The use of special kaolins to improve the printability of paper forrotogravure process has the disadvantage that they usually exhibit poorTheological characteristics, therefore precluding the possibility towork with a high content of solids and/or at high speeds during theirapplication.

As a conclusion, we can say that a good paper coating formulation is theresult of a compromise: every improvement of the printability, even whenlimited, causes automatically at least one (but often all) the followingdrawbacks: higher costs, rheological worsening, diminishment of thewhiteness, worsening of the printing machine operating level.

Any improvement of the whole of these characteristics is still theobject of many research projects.

In the state of the art many methods have been described which aresubstantially directed to the improvement of the printabilty of paper(we cite, by way of example, US 20010051687, U.S. Pat. No. 5,085,707,U.S. Pat. No. 4,908,240), but none of the proposed methodssatisfactorily solves the above mentioned problems in the case of thepaper for rotogravure printing.

It is an object of the present invention a procedure to improve theprintabilty of paper, and in particular the printability of the paperdestined for high speeds printing processes, such as the paper forrotogravure processes.

It has now surprisingly been found that paper possess improvedprintability when treated with the paper coating formulation of theinvention, if we compare its printability with the one of the papercoated with the common aqueous suspensions comprising pigments,adhesive, and dispersing agents.

According to a fundamental aspect of the present invention, the papercoating formulations for rotogravure processes contain:

-   -   a. 100 parts by weight of finely divided pigments;    -   b. from 0.001 to 5 parts, preferably from 0.01 to 1, more        preferably from 0.02 to 0.8 by weight, of one or more substances        selected from the group consisting of: mono-alkylsulfosuccinate;        di-alkylsulfosuccinates; sulfosuccinic acid mono-esters of        ethoxylated and/or propoxylated fatty alcohols; sulfosuccinic        acid di-esters of ethoxylated and/or propoxylated fatty        alcohols;    -   c. from 3 to 15 parts by weight of a polymeric acrylic binder;    -   d. from 0.005 to 0.4 parts by weight of a dispersant. According        to a preferred aspect of the invention the mono- and        di-alkylsulfosuccinate utilisable are mono- or di-C₂-C₁₆ linear        or branched alkylsulfosuccinates; more preferably the        di-alkylsulfosuccinate is dioctylsulfosuccinate.

The sulfosuccinic acid mono- and di-esters of ethoxylated and/orpropoxylated fatty alcohols useful for the realization of the presentinvention are ethoxylated and/or propoxylated with from 1 to 50,preferably from 20 to 40 moles of oxide.

With the term fatty alcohol in the present text we mean C₈-C₃₀ linear orbranched alkyl alcohols.

The finely divided pigments, preferably having from 40 to 90% of theparticles finer than 2 μm, are the one normally employed in the coatingof paper for rotogravure printing, and particularly kaolins, calciumcarbonate, talc, titanium dioxide, barium sulfate, gypsum.

The mixture of finely divided pigments preferably contains at least 30%by weight of kaolin for rotogravue printing having from 40 to 70% of theparticles finer than 2 μm.

Among the polymeric acrylic binder preferred for the realisation of theinvention we cite the polymers of acrylic or methacrylic acid esters,the copolymers of acrylic monomers and vinyl acetate, styrene, butadieneor mixture thereof; among the preferred dispersants we cite the aqueoussolutions of sodium or ammonium polyacrylates.

In addition to the above cited substances, the paper coating formulationfor rotogravure printing of the invention preferably contain from 0.3 to2 parts by weight of calcium stearate.

The paper coating formulation is normally applied to the sheet in theform of aqueous dispersion further containing thickeners and, possibly,anti-foaming agents; for the realisation of the present invention,preferably, the aqueous dispersion contains from 40 to 70% by weight ofthe paper coating formulation for rotogravure printing above describedand from 30 to 60% by weight of water.

A further advantage of the invention is that the aqueous dispersions ofthe paper coating formulation do not necessarily need the presence ofanti-foaming agents; or, at least, the need of said agents in order toavoid the formation of foams which reduce the operating speed of thecoating machines and adversely affect the printability of paper, issubstantially diminished.

It is a further object of the present invention the paper forrotogravure printing processes that is coated with from 4 to 15 g/m2,preferably from 6 to 10 g/m², of a thin layer of the paper coatingformulation above described.

EXAMPLE 1

Five aqueous dispersions of paper coating formulations are prepared withthe following ingredients:

-   -   delaminated pre-dispersed kaolin, particle size 75% finer than 2        μm (Kaolin A);    -   delaminated kaolin for rotogravure printing, particle size 50%        finer than 2 μm (Kaolin B);    -   78% by weight dispersion of calcium carbonate GCC, particle size        about 90% finer than 2 μm (Carbonate A);    -   Reotan A, dispersant based on sodium polyacrylate commercialised        by Lamberti SpA (Italy);    -   Acronal 500 D, an acrylic polymeric binder for rotogravure        printing commercialised by BASF;    -   Lamkote, calcium stearate in emulsion commercialised by Lamberti        SpA (Italy);    -   Carbocel MM3, carboxymethylcellulose commercialised by Lamberti        SpA (Italy) having Brookfield viscosity of 20-50 mPa*s at 60 rpm        in a 2% by weight aqueous solution;    -   Viscolam 30, a polyacrylic thickener, commercialised by Lamberti        SpA (Italy).

Preparation of the “Base” Aqueous Dispersion.

(The “base” aqueous dispersion will then be used to obtain the fiveaqueous dispersion of the paper coating formulations to be tested, toavoid experimental errors and to guarantee the comparability of theresults)

A 68% by weight dispersion is prepared adding to 296.5 g of water, undervigorous stirring, 630 g of Kaolin A, 0.13 g of Reotan A and 25% aq.NaOH to obtain a pH of 8.5-9. The dispersion is obtained stirring with acaowles at 1000 rpm for 30 minutes.

Then, in the same manner a second dispersion (having a content of solidsof 68% by weight) is prepared with: 487.6 g of water, 990.0 g of KaolinB, 4.0 g of Reotan A and 25% aq. NaOH.

With the thus obtained two dispersions of kaolins the “base” aqueousdispersion is prepared as follows.

The two dispersions of kaolins are mixed with a caowles at 1000 rpm;then 230.8 g of Carbonate A are added and the mixture is homogenised bystirring at 1000 rpm for 30 minutes.

Then the stirring speed is diminished to 700 rpm and 288 g of Acronal500 D are added; after 10 minutes stirring 27 g of Lamkote are added;after 5 minutes stirring 9 g of Carbocel MM3 (previously prepared as a5% wlw aqueous solution); finally, 18 g of Viscolam 30 are added, alwaysunder stirring. The pH of the dispersion is then 8.6.

The dry fraction is then determined with a Mettler-Toledo thermo-balanceset at 105° C. (result emitted after 3 minutes of constant weight) andwater is added to dispersion until the desired value of dry fraction(60.8%) is obtained.

Finally the “base” aqueous dispersion is homogenised by stirring at 700rpm for 15 minutes.

Five portions (each weighing 400 g) of the “base” aqueous dispersion aretaken to prepare the aqueous dispersions of the paper coatingformulations used for the following comparative tests.

The first portion is used as such as the aqueous dispersion of thereference paper coating formulation (Dispersion 1).

The Dispersion 2, 3, 4 and 5 are prepared by respectively adding to theremaining four portions:

0.1 5 g of dioctylsulfosuccinate (Dispersion 2);

0.78 g of dioctylsulfosuccinate (Dispersion 3);

0.18 g of the sulfosuccinic acid mono-ester with cetylstearyl alcohol 30moles propoxylated, 4 moles ethoxylated (Dispersion 4);

0.89 g of the sulfosuccinic acid mono-ester with cetylstearyl alcohol 30moles propoxylated, 4 moles ethoxylated (Dispersion 5);

The five dispersions of the paper coating formulations are stirred for15 minutes and mantained at 25° C.; Dispersion 1 too, even if it doesnot containing any additional ingredient, is stirred for 15 minutesbefore being tested, to guarante the comparability of the results.

The Brookfield viscosity of the Dispersion 1-5 is 1040 m*Pas (100 rpm).

Coating and Evaluation of Printability.

The paper coating is performed with the Dispersion 1-5 on industrialpaper rotogravure sheets of 40 g/m².

A coating bar machine is used, suitable for the plane coating of A4sheets; the machine has a set of bars wound by threads having differentdiameters, allowing to vary the volume of the coat by changing thedosing bar; it is also possible to vary the speed of the moving bar tomodify the amount of coat applied.

The coating machine, after a series of tests made to optimize theprocedure, is regulated to dose 8 g/m² of dry coat on the desiredsupport. As the Dispersions 1-5 have the same contents of solids and thesame rheology the regulation of the coating machine is the same in alltests and the machine constantly deposits 8 g/m² of dry coat.

Immediately after the coating, the sheets are dried with air for 15″ at120° C., and then an maintained for 2 minutes at 110° C.

The coated sheets are allowed to stay in a conditioned room for 24 hoursat 21° C. and 50% of relative humidity; then they are calendered withthe temperature of the rolls set at 50° C., linear pressure=67.5 Kg/cm,4 nip, and contacting the coated side of the sheets on the steel roll.After being calendered the sheets are again conditioned at 21° C. and50% of relative humidity.

The rotogravure printability is evaluated with Heliotest, a universallyknown method which is used both in the paper industry for qualitycontrol and in the research laboratories to evaluate the quality ofpaper for rotogravure printing.

The printing pressure is set at 55 Kg for all tests; to minimise errorsnine Heliotest values are taken from as many samples, cut from the fivesheets coated with the Dispersions 1-5 (1-5 in Table 1); the valuereported in Table 1 (Missing Dots—mm) is the average of the nineHeliotest values. TABLE 1 1* 2 3 4 5 Heliotest 20° 63.0 73.2 69.5 69.076.1*(comparative)

EXAMPLE 2

Five aqueous dispersions of paper coating formulations are prepared withthe following ingredients:

-   -   delaminated pre-dispersed kaolin for rotogravure printing,        particle size 62% finer than 2 μm (Kaolin C);    -   delaminated kaolin, particle size 68% finer than 2 μm (Kaolin        D);    -   Reotan A, dispersant based on sodium polyacrylate commercialised        by Lamberti SpA (Italy);    -   78% by weight dispersion of calcium carbonate GCC, particle size        about 90% finer than 2 μm (Carbonate A);    -   Acronal 500 D, an acrylic polymeric binder for rotogravure        printing commercialised by BASF;

Preparation of the “Base” Aqueous Dispersion.

(The “base” aqueous dispersion will then be used to obtain the fiveaqueous dispersion of the paper coating formulations to be tested, toavoid experimental errors and to guarantee the comparability of theresults)

A 68% by weight dispersion is prepared adding to 225.9 g of water, undervigorous stirring, 480 g of Kaolin C, 0.12 g of Reotan A and 25% aq.NaOH to obtain a pH of 8.5-9. The dispersion is obtained stirring with acaowles at 1000 rpm for 30 minutes.

Then, in the same manner a second dispersion (having a content of solidsof 63% by weight) is prepared with: 480 g of Kaolin, 1.2 g of Reotan Aand 25% aq. NaOH.

Then, in the same manner a third dispersion (having a content of solidsof 60% by weight) is prepared with: 180 g of talc, 0.36 g of Reotan Aand 25% aq. NaOH.

With the thus obtained three dispersions the “base” aqueous dispersionis prepared as follows.

The three dispersions are mixed with a caowles at 1000 rpm; then 76.9 gof Carbonate A are added and the mixture is homogenised by stirring at1000 rpm for 30 minutes.

Then the stirring speed Is diminished to 700 rpm and 108 g of Acronal500 D are added; after 10 minutes stirring 18 g of Lamkote are added;finally, 6 g of Viscolam 30 are added, always under stirring. The pH ofthe dispersion is then 8.5.

The dry fraction is then determined with a Mettler-Toledo thermo-balanceset at 105° C. (result emitted after 3 minutes of constant weight) andwater is added to dispersion until the desired value of dry fraction(52.7%) is obtained.

Finally the “base” aqueous dispersion is homogenised by stirring at 700rpm for 1 5 minutes.

Four portions (each weighing 400 g) of the “base” aqueous dispersion aretaken to prepare the aqueous dispersions of the paper coatingformulations used for the following comparative tests.

The first portion is used as such as the aqueous dispersion of thereference paper coating formulation (Dispersion 6).

The Dispersion 7, 8 and 9 are prepared by respectively adding to theremaining three portions:

0.14 g of dioctylsulfosuccinate (Dispersion 7);

0.16 g of the sulfosuccinic acid mono-ester with cetylstearyl alcohol 30moles propoxylated, 4 moles ethoxylated (Dispersion 8);

0.80 g of the sulfosuccinic acid mono-ester with cetylstearyl alcohol 30moles propoxylated, 4 moles ethoxylated (Dispersion 9);

The four dispersions of the paper coating formulations are stirred for15 minutes and mantained at 25° C.; Dispersion 6 too, even if it doesnot containing any additional ingredient, is stirred for 15 minutesbefore being tested, to guarante the comparability of the results.

Coating and Evaluation of Printability.

The paper coating is performed with the Dispersion 6-9 on industrialpaper rotogravure sheets of 40 g/m².

The same coating bar machine used for Example 1.

The coating machine, after a series of tests made to optimize theprocedure, is regulated to dose 8 g/m² of dry coat on the desiredsupport. The regulation of the coating machine is the same in all thefollowing tests.

Immediately after the coating, the sheets are dried with air for 15″ at120° C, and then are maintained for 2 minutes at 110° C.

The coated sheets are allowed to stay in a conditioned room for 24 hoursat 21° C. and 50% of relative humidity; then they are calendered withthe temperature of the rolls set at 50° C., linear pressure=67.5 Kg/cm,4 nip, and contacting the coated side of the sheets on the steel roll.

After being calendered the sheets are again conditioned at 21° C. and50% of relative humidity.

The rotogravure printability is evaluated with Heliotest.

The printing pressure is set at 55 Kg for all tests; to minimise errorseight Heliotest values are taken from as many samples, cut from the fivesheets coated with the Dispersions 6-9 (6-9 in Table 2); the valuereported in Table 2 (Missing Dots—mm) is the average of the eightHeliotest values. TABLE 2 6* 7 8 4 Heliotest 20° 62 68 63 75*(comparative)

1-16. (canceled)
 17. A paper coating formulation useful for rotogravureprinting processes comprising: a. 100 parts by weight of finely dividedpigments; b. from 0.001 to 5 parts by weight of a substance selectedfrom the group consisting of: mono-alkylsulfosuccinate;di-alkylsulfosuccinates; sulfosuccinic acid mono-esters of ethoxylatedand/or propoxylated fatty alcohols; sulfosuccinic acid di-esters ofethoxylated and propoxylated fatty alcohols; and mixtures thereof; c.from 3 to 15 parts by weight of a polymeric acrylic binder; and d. from0.005 to 0.4 parts by weight of a dispersant.
 18. The paper coatingformulation according to claim 17 additionally comprising from 0.3 to 2parts by weight of calcium stearate.
 19. The paper coating formulationaccording to claim 17 wherein the sulfosuccinic acid mono- and di-estersof ethoxylated and/or propoxylated fatty alcohols are ethoxylated and/orpropoxylated with from 1 to 50 moles of oxide.
 20. The paper coatingformulation according to claim 19 wherein the sulfosuccinic acid mono-and di-esters of ethoxylated and/or propoxylated fatty alcohols areethoxylated and/or propoxylated with from 20 to 40 moles of oxide. 21.The paper coating formulation according to claim 17 wherein themono-alkylsulfosuccinate and di-alkylsulfosuccinate are mono- ordi-C₂-C₁₆ linear or branched alkylsulfosuccinates.
 22. The paper coatingformulation according to claim 17 wherein the di-alkylsulfosuccinate isdioctylsulfosuccinate.
 23. The paper coating formulation according toclaim 17 wherein the substance selected from the group consisting of:mono-alkylsulfosuccinate; di-alkylsulfosuccinates; sulfosuccinic acidmono-esters of ethoxylated and/or propoxylated fatty alcohols;sulfosuccinic acid di-esters of ethoxylated and/or propoxylated fattyalcohols; and mixtures thereof; is present at from 0.01 to 1 parts byweight.
 24. The paper coating formulation according to claim 17 whereinthe substance selected from the group consisting of:mono-alkylsulfosuccinate; di-alkylsulfosuccinates; sulfosuccinic acidmono-esters of ethoxylated and/or propoxylated fatty alcohols;sulfosuccinic acid di-esters of ethoxylated and/or propoxylated fattyalcohols; and mixtures thereof; is present at from 0.02 to 0.8 parts byweight.
 25. The paper coating formulations according to claim 17 whereinthe finely divided pigments have from 40 to 90% of the particles finerthan 2 μm.
 26. The paper coating formulation according to claim 25wherein the mixture of finely divided pigments contains at least 30% byweight of kaolin for rotogravure printing having from 40 to 70% of theparticles finer than 2 μm.
 27. An aqueous dispersion useful for thecoating of rotogravure printing paper comprising from 40 to 70% byweight of the paper coating formulation according to claim 1 and from 30to 60% by weight of water.
 28. The aqueous dispersion of claim 27additionally comprising from 0.3 to 2 parts by weight of calciumstearate based upon the weight of the paper coating formulationaccording to claim
 17. 29. A paper useful for rotogravure printingprocesses characterized by the fact that it is coated with from 4 to 15g/m² of a thin layer of the paper coating formulation of claim
 17. 30.The paper according to claim 29, characterized by the fact that it iscoated with from 6 to 10 g/m² of a thin layer of the paper coatingformulation of claim
 17. 31. A method to improve the printability ofrotogravure paper comprising the step of treating the paper with a papercoating formulations containing: a. 100 parts by weight of finelydivided pigments; b. from 0.001 to 5 parts by weight of a substanceselected from the group consisting of: mono-alkylsulfosuccinate;di-alkylsulfosuccinates; sulfosuccinic acid mono-esters of ethoxylatedand/or propoxylated fatty alcohols; sulfosuccinic acid di-esters ofethoxylated and propoxylated fatty alcohols; and mixtures thereof; c.from 3 to 15 parts by weight of a polymeric acrylic binder; and d. from0.005 to 0.4 parts by weight of a dispersant.
 32. The method toaccording to claim 31, wherein the paper is coated with from 4 to 15g/m² of the paper coating formulation.
 33. The method according to claim31, the paper coating formulation additionally comprising from 0.3 to 2parts by weight of calcium stearate.
 34. A method to improve theprintability of rotogravure paper comprising the step of treating thepaper with an aqueous dispersion consisting of from 30 to 60% by weightof water and from 40 to 70% by weight of a paper coating formulationcontaining: a. 100 parts by weight of finely divided pigments; b. from0.001 to 5 parts by weight of a substance selected from the groupconsisting of: mono-alkylsulfosuccinate; di-alkylsulfosuccinates;sulfosuccinic acid mono-esters of ethoxylated and/or propoxylated fattyalcohols; sulfosuccinic acid di-esters of ethoxylated and propoxylatedfatty alcohols; and mixtures thereof; c. from 3 to 15 parts by weight ofa polymeric acrylic binder; and d. from 0.005 to 0.4 parts by weight ofa dispersant.
 35. The method according to claim 34, the paper coatingformulation additionally comprising from 0.3 to 2 parts by weight ofcalcium stearate.
 36. A paper coating formulation useful for rotogravureprinting processes comprising: a. 100 parts by weight of finely dividedpigments; b. from 0.001 to 5 parts by weight of a substance selectedfrom the group consisting of: mono-alkylsulfosuccinate;di-alkylsulfosuccinates; sulfosuccinic acid mono-esters of ethoxylatedand/or propoxylated fatty alcohols; sulfosuccinic acid di-esters ofethoxylated and propoxylated fatty alcohols; and mixtures thereof; c.from 3 to 15 parts by weight of a polymeric acrylic binder; d. from0.005 to 0.4 parts by weight of a dispersant; and e. from 0.3 to 2 partsby weight of calcium stearate.